1. Field of the Invention
The present invention relates to a device and a method for spraying a solid lubricant toward an injection sleeve and a spool bushing in a die-casting machine.
2. Description of Related Art
Known in the prior art is a die-casting machine, which has a fixed mold and a platen, which are provided with a spool bushing and an injection sleeve, respectively. The spool bushing and the injection sleeve form aligned cylindrical bores opening into a cavity for molding a product. A plunger is slidably inserted into the cylinder bores for injecting the molten metal into the cavity. In order to reduce the frictional force between the plunger and the injection sleeve and the spool bushing a device is provided for supplying a lubricant onto the cylindrical surfaces of the injection sleeve and the spool bushing. The coating of the lubricant serves, also, as a heat insulating layer for maintaining a desired high temperature of the molten metal material.
A liquid lubricant, such as an oil or an aqueous wax, has, heretofore, usually been used. There is, however, a recent need to obtain a die casting product of a precision as high as possible. In order to do this, an ultra-low-speed injection method is proposed, which is effective to prevent gas from being entrained when the molten metal material is injected into the cavity. In this ultra-low-speed injection method, the speed of the movement of the plunger is as low as between 3 and 5 cm/second, so that a laminar flow of the molten metal material is obtained which prevents gas from being entrained. However, the ultra-low-speed injection method causes the molten metal material to be gradually supplied, from the injection sleeve and the spool bushing, to the cavity. As a result, contact between molten metal material, at a high temperature, with the injection sleeve and the spool bushing is prolonged, which causes the lubricant to fail to thermally insulate the molten metal since the liquid lubricant is relatively easily vaporized. As a result, a need has arisen to employ a solid lubricant formed as fine solid particles dispersed in a liquid, instead of liquid lubricant, in order to improve the thermal insulation of the molten metal.
However, the conventional device for supplying the lubricant suffers from following drawbacks if it is combined with the ultra-low-speed injection method while using the solid lubricant. Namely, in the prior art system, the supply pipe is merely employed for supplying the lubricant to the injection sleeve at a location adjacent its inlet. A supply of a solid lubricant by means of this type of apparatus may cause the supplied solid lubricant to be locally concentrated in the injection sleeve and the spool bushing. Namely, unlike the liquid lubricant, the solid lubricant is not evenly spread in the injection sleeve and the spool bushing, it is difficult to obtain a uniform heat insulating coating of the lubricant. Due to the lack of a uniform coating of the lubricant, a reduction in the temperature of the molten metal occurs during the injection of the molten metal, which causes the molten metal to fail to flow smoothly into the cavity, so that a desired shape of the product after removal of the mold cannot be obtained at locations where the product is thin or at the downstream end of the flow of the molten metal in the cavity. Furthermore, at locations where the lubricant coating is thin or absent, the molten material in the injection sleeve and the spool bushing are subjected to rapid cooling. Such rapidly cooled parts in a die-cast product reduces the strength of the product.
Furthermore, the prior art construction employing a supply tube for supplying the lubricant cannot supply a thin uniform coating of the solid lubricant. As a result, portions with excessive amount of the lubricant are created in the coating, which causes the lubricant to be entrained in the molten metal, which results in voids in the molded product.
Finally, the solid lubricant is dispersed in a liquid. As a result, the small lubricant particles are apt to accumulate at the outlet end of the lubricant supply pipe. As a result, longer the lubricant supply pipe is used, smaller the effective inner diameter of the lubricant supply pipe. As a result, a reduction in the amount of the solid lubricant supplied occurs, which causes the above mentioned drawbacks to occur earlier. Furthermore, a reduction in the supply of the lubricant can cause the molten metal to adhere to the injection sleeve and/or the spool bushing, which prevents the injection plunger from being smoothly moved.